The present invention concerns network analyzers and pertains particularly to obtaining calibration parameters for a three-port device under test.
A network analyzer typically integrates a synthesized signal source with built-in signal separation devices, receivers, a display and a processor.
Measurement calibration is a process that characterizes the systematic errors of a network analyzer. This information can be used to improve measurement accuracy by using error correction arrays during signal processing to compensate for systematic measurement errors. Measurement calibration is also called Cal, a short form of calibration. Error correction is also called accuracy enhancement. Measurement errors are classified as random and systematic errors. Random errors, such as noise and connector repeatability are non-repeatable and not correctable by measurement calibration and error correction.
Systematic errors, such as tracking and crosstalk, are the most significant errors in most network analyzer measurements. Systematic errors are repeatable and for the most part correctable, though small residual errors may remain. These systematic errors may drift with time and temperature and therefore require new measurement calibrations to maintain error corrected measurement accuracy.
Systematic errors are due to system frequency response, isolation between the signal paths, and mismatch in the test setup. Frequency response errors (transmission and reflection tracking) result from the difference of the test signal path and receiver with respect to the reference signal path and receiver that are a function of frequency.
Isolation errors result from energy leakage between signal paths in measurements. This leakage is due to crosstalk. In reflection measurements, the leakage is also due to imperfect directivity. Directivity is the ability of the signal separation devices to separate forward traveling signals from reverse traveling signals.
Mismatch errors result from differences between the port impedance of the device under test (DUT) and the port impedance of the network analyzer. Source match errors are produced on the source (network analyzer OUT) side of the DUT. Load match errors are produced on the load (network analyzer IN) side. If the DUT is not connected directly to the ports, the mismatch errors due to cables, adapters, etc. are considered part of the source or load match errors.
The network analyzer has several methods of measuring and compensating for these test system errors. Each method removes one or more of the systematic errors using equations derived from an error model. Measurement of high quality standards (for example, short, open, load, through) allows the network analyzer to solve for the error terms in the error model. The accuracy of the calibrated measurements is dependent on the quality of the standards used and the stability of the measurement system. Since calibration standards are very precise, great accuracy can be achieved.
To perform a transmission calibration, at least four measurement standards are utilized: for example, an open, a short, a load, and a through cable. The network analyzer measures each standard across a defined frequency band using a pre-defined number of points. The measurement of these standards is used to solve for the error terms in the error model and to remove systematic errors caused by transmission frequency response, load match and source match.
To perform a reflection calibration, a one-port calibration is performed using at least three measurement standards, such as an open, a short, and a load. The network analyzer measures each standard across a predefined frequency band using a pre-defined number of points. The measurements of these standards are used to solve for the error terms in the error model and to remove systematic errors caused by directivity, source match and reflection frequency response.
For further information about calibration of network analyzers, see for example, the HP 8712C and HP 8714C RF Network Analyzer User's Guide, Part No. 08712-90056, October, 1996, pp. 6-1 through 6-14, available from Agilent Technologies, Inc.
In order to reduce the time required for calibration various systems have incorporated some automated features. For example U.S. Pat. No. 5,434,511, U.S. Pat. No. 5,467,021, U.S. Pat. No. 5,537,046, U.S. Pat. No. 5,548,221, U.S. Pat. No. 5,552,714 and U.S. Pat. No. 5,578,932 discuss electronic calibration accessories that perform computer-assisted calibrations with electronic standards, making the calibration process less time-consuming and error-prone. When using these electronic calibration accessories it is necessary to manually connect a module to the measurement ports. U.S. Pat. No. 5,587,934 also sets out an electronic calibration module that uses manual connections. U.S. Pat. No. 5,548,538 discloses a technique for including calibrations internal to the network analyzer.
When measuring a three-port device using a two-port network analyzer, the device needs to be measured three times. Typically, it has been necessary to move cables in order to accommodate a port orientation to the device that is different for each calibration measurement. Because test port cable characteristics change with cable movements, calibration accuracy is reduced. A vector network analyzer (VNA) two-port calibration method known as “unknown through” calibration is used to minimize cable movement and connections during calibration of a three-port device. See, for example, A. Ferrero., “Two-Port Network Analyzer Calibration Using an Unknown “Thru””, IEEE Microwave and Guided Wave Letter, Vol. 2, No. 12, December 1992. pp. 505-507. Using this method, the test port cables can be positioned to align with the desired measurement ports of the three-port device. The test port connectors are configured to mate with the measurement ports of the three-port device. Then, the appropriate one-port calibration standards are connected to each test port and measured. A calibration module may be used to reduce the number of connections. The three-port device is then connected between the test ports as the “unknown thru” with the third port terminated by a load or equivalent, to complete the VNA calibration.
A minimum of three different terminations is connected to the third port to obtain the data required to extract the three-port S-parameters. Three different known standards may be used; however, this disconnection and reconnection is time consuming and connection and disconnection of terminations may lead to differences in measurement that are not repeatable.